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#include <iostream>
 
template<typename T>
struct Cloneable
{
	virtual T *Clone() const
	{
		return new T(*dynamic_cast<const T*>(this));
	}
protected:
	Cloneable(){}
	Cloneable(const Cloneable &){}
	virtual Cloneable &operator=(const Cloneable &){ return*this; }
	virtual ~Cloneable(){}
};
 
struct A : Cloneable<A>
{
	A() : x(1) {}
	A(const A &f) : x(f.x) {}
	A(int v) : x(v) {}
	virtual A &operator=(const A &f)
	{
		x = f.x;
		return*this;
	}
 
	virtual A *Clone() const = 0;
	virtual void Print() const
	{
		std::cout << "x=" << x << std::endl;
	}
 
	virtual ~A(){}
private:
	int x;
};
 
struct B : A, Cloneable<B>
{
	B() : q(2) {}
	B(const B &f) : A(f), q(f.q) {}
	B(int v) : A(v-1), q(v) {}
	virtual B &operator=(const B &f)
	{
		A::operator=(f);
		q = f.q;
		return*this;
	}
	virtual A &operator=(const A &f)
	{
		A::operator=(f);
		const B *b = dynamic_cast<const B*>(&f);
		if(b)
		{
			q = b->q;
		}
		return*this;
	}
 
	using Cloneable<B>::Clone;
	virtual void Print() const
	{
		std::cout << "q=" << q << ", ";
		A::Print();
	}
 
	~B(){}
private:
	int q;
};
 
int main()
{
	B b1 (7), b2, *b3 (0);
	A &a1 (b1), &a2 (b2), *a3 (0);
 
	b1.Print();
	b2.Print();
 
	a1.Print();
	a2.Print();
 
	b3 = b1.Clone();
	b3->Print();
	delete b3;
 
	a3 = a1.Clone();
	a3->Print();
	delete a3;
 
	b2 = b1;
	b2.Print();
 
	a3 = new B(9);
	a2 = *a3;
	a2.Print();
	b2.Print();
}

prog.cpp: In function ‘int main()’:
prog.cpp:75: error: cannot declare variable ‘b1’ to be of abstract type ‘B’
prog.cpp:40: note:   because the following virtual functions are pure within ‘B’:
prog.cpp:28: note: 	virtual A* A::Clone() const
prog.cpp:75: error: cannot declare variable ‘b2’ to be of abstract type ‘B’
prog.cpp:40: note:   since type ‘B’ has pure virtual functions
prog.cpp:95: error: cannot allocate an object of abstract type ‘B’
prog.cpp:40: note:   since type ‘B’ has pure virtual functions
prog.cpp: In member function ‘T* Cloneable<T>::Clone() const [with T = B]’:
prog.cpp:84:   instantiated from here
prog.cpp:8: error: cannot allocate an object of abstract type ‘B’
prog.cpp:40: note:   since type ‘B’ has pure virtual functions
prog.cpp: In member function ‘T* Cloneable<T>::Clone() const [with T = A]’:
prog.cpp:99:   instantiated from here
prog.cpp:8: error: cannot allocate an object of abstract type ‘A’
prog.cpp:18: note:   because the following virtual functions are pure within ‘A’:
prog.cpp:28: note: 	virtual A* A::Clone() const

Standard output is empty